I think the reason is that the angle from behind the post does not quite show it well enough. By that I mean that you cannot quite see if it is down and whether or not it's before the line. You have to also look at it from the TMO's point of view. If he is not 100% certain then he can't give it.

Personally I don't think it was a try. However, even if Strettle and the rest of England do, it couldn't have been awarded based on what the question was. Walsh asked 'try or no try' which means that the TMO has to find a clear shot of the try being scored. Despite what many people will say, there was no clear shot of the ball being grounded. There were a few which looked like it could have been but none that gave a clear shot for a try therefore, as the TMO says, it was inconclusive.

However, I would also like to add that England played much much better than they have this championship and a draw would have been a fair result. England supporters should not be too upset by that loss as it is a learning curve for the team and also, given a few more months, England could do some real damage.

I agree with everything everyone has said above. Not to mention that Armitage probably milked it a bit. Even if he didn't he can't complain. He's only getting a fraction of what deals out.
That being said, very daft to do it right in front of the touch judge.